High styrene and high vinyl SSBR is difficult to produce due to the kinetics of copolymerization. Typically, polar agents known as randomizers are added to the polymerization system in order to achieve random styrene incorporation.
The use of certain randomizers can result in high vinyl SSBR having a low block styrene content (>6 successive units of styrene) below 10%. Long block styrene can worsen hysteresis as reported, for example, by S. Futamura and G. Day who observed a worsening by about 18% of the tan delta at 60° C. when increasing block styrene content from 2 to about 7% (Kautschuk Gummi Kunststoffe, 1987, 40, No. 1, 39-43) in a carbon black-filled compound. By contrast, incorporation of small styrene blocks can result in improved abrasion and tensile strengths, particularly in silica compounds, as reported by I. Hattori et al, (143rd Meeting of the Rubber Division of the ACS, Spring 1993, paper 22).
In certain applications, it would be desirable to achieve a high styrene high vinyl SSBR having a defined incorporation of styrene as blocks of 4-6 units.